Aromatics alkylation process

ABSTRACT

The invention relates to a process for producing alkylated aromatics, preferably ethylbenzene, in a multiple bed reactor in which at least two catalysts, each comprising a molecular sieve, are used in sequential beds. The first alkylation catalyst is selected to have a higher activity or alpha value than the subsequent alkylation catalyst.

FIELD OF THE INVENTION

The present invention relates to a process for improving the efficiencyand reducing certain byproducts in alkylation of aromatic compounds toproduce mono-alkylaromatic compounds. In particular, vapor phasealkylation of benzene to produce ethylbenzene can be accomplished withincreased ethylbenzene purity and reduced ethylene and benzene loss tobyproduct formation. Alternatively, the capacity of an existing processcan be increased while maintaining product specifications.

BACKGROUND OF THE INVENTION

A variety of processes for converting aromatics in the presence ofmolecular sieve catalysts are known in the chemical processing industry.Aromatic conversion reactions include alkylation and transalkylation toproduce alkylaromatics such as ethylbenzene (EB), ethyltoluene, cumeneand higher aromatics. An alkylation reactor which produces a mixture ofmono- and poly-alkylaromatic compounds may be linked in some way with atransalkylation reactor to maximize the net production ofmono-alkylaromatic compounds. Such alkylation and transalkylationconversion processes can be carried out in the liquid phase, in thevapor phase, or under conditions in which both liquid and vapor phasesare present. The preferred catalysts and the byproduct formation differwith the severity of reaction conditions and the phase conditions inwhich the reaction is carried out.

In efforts to improve commercial alkylation operations, emphasis isplaced not only on the conversion efficiency of the catalyst but also onthe selectivity of the catalyst, including reduced production of certainbyproducts. For example, in the manufacture of ethylbenzene, ethyleneand benzene are introduced into an alkylation reactor in the presence ofvarious catalysts. Some of the byproducts include diethylbenzenes,xylenes, propylbenzene, cumene, butylbenzene, and other componentsreferred to collectively as heavies. Production of unwanted byproductsincreases feedstock usage as well as the cost of separating suchunwanted byproducts. Byproducts which are not removed can materiallyimpact the efficiency of downstream operations, such as thedehydrogenation of EB to form styrene monomer.

It has been shown that zeolites like ZSM-5 show high activity andselectivity for vapor phase alkylation of benzene with ethylene and thatcatalysts of this type in the acid form remain active for unusually longperiods between regenerations. Discussion of acid zeolite ZSM-5 forvapor phase alkylation is provided in U.S. Pat. No. 3,751,506, which isherein fully incorporated by reference and which describes control ofthe exothermic heat of reaction by conducting the reaction in a seriesof reactors with intermediate cooling and addition of ethylene betweenstages.

Another process for vapor phase alkylation is described in U.S. Pat. No.4,107,224, which is herein fully incorporated by reference. Benzene anddilute ethylene are reacted in vapor phase over a solid porous catalystselected from ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38, and other similarmaterials in a series of reaction zones with intermediate injection ofcold reactants and diluent to control temperature.

U.S. Pat. No. 6,090,991, which is herein fully incorporated byreference, describes vapor phase ethylbenzene production in which afeedstock containing benzene and ethylene is applied to an alkylationreaction zone having at least one catalyst bed containing a monoclinicsilicalite catalyst having a weak acid site concentration of less than50 micromoles per gram.

U.S. Pat. No. 6,057,485, which is herein fully incorporated byreference, describes vapor phase ethylbenzene production by alkylationover a split load of monoclinic silicalite alkylation catalysts havingdifferent silica/alumina ratios. A feedstock containing benzene andethylene is applied to a multi-stage alkylation reaction zone having aplurality of series-connected catalyst beds. At least one catalyst bedcontains a first monoclinic silicalite catalyst having a silica/aluminaratio of at least 275. At least one other catalyst bed contains a secondmonoclinic silicalite catalyst having a silica/alumina ratio of lessthan about 275.

U.S. Pat. No. 5,998,687, which is herein fully incorporated byreference, describes ethylbenzene production by alkylation over astacked reactor loaded with zeolite beta followed by zeolite Y to reduceoverall flux oil production.

A disadvantage of vapor phase alkylation reactions is the formation ofpolyalkylated byproducts. While the art currently provides for varioustransalkylation processes to handle some of the alkylation byproductssuch as diethylbenzene, it would be desirable to reduce the productionof byproducts, especially byproducts that are not easily handled in analkylation/transalkylation process. It would also be desirable to reducethe quantity of reactants consumed in production of byproducts

Recently, catalysts have been developed which allow the alkylationreactions to be carried out in the liquid phase alkylation at relativelymild reaction conditions. The reduced temperature associated withoperating in the liquid phase allows for a significant reduction inundesirable by-products.

In existing facilities designed for vapor phase reactions, it can becost-prohibitive to retrofit for a liquid phase operation unless asubstantial increase in production capacity is required. Improvedcatalysts allowing lower temperature operation in such vapor phasefacilities are highly desirable.

SUMMARY OF THE INVENTION

In one embodiment, this invention is a process for alkylating anaromatic hydrocarbon reactant with an alkylating agent to produce analkylated aromatic product, said process comprising:

-   -   (a) introducing said aromatic hydrocarbon reactant and said        alkylating agent into a reactor unit containing a plurality of        sequentially arranged beds comprised of a first bed containing a        first catalyst effective for alkylating said aromatic        hydrocarbon reactant and a second bed downstream from said first        bed and containing a second catalyst effective for alkylating        said aromatic hydrocarbon reactant and having less catalytic        activity than said first catalyst;    -   (b) alkylating in said first bed under alkylation conditions        said aromatic hydrocarbon reactant with said alkylating agent to        form a first effluent comprising a mono-alkylaromatic compound,        an unreacted portion of the aromatic hydrocarbon reactant, and        polyalkylated aromatic compounds,    -   (c) alkylating in said second bed under alkylation conditions at        least a portion of said unreacted aromatic hydrocarbon reactant        present in said effluent with said alkylating agent to form a        product effluent, and    -   d) removing said product effluent from said reactor unit, said        product effluent comprising a mono-alkylaromatic compound, an        unreacted portion of the aromatic hydrocarbon reactant, and        polyalkylated aromatic compounds.

In another embodiment, this invention can be a process for thevapor-phase ethylation of benzene comprising

-   -   a) providing a multi-stage alkylation reaction zone having a        plurality of series-connected catalyst beds, at least one of the        series-connected catalyst beds containing a first alkylation        catalyst comprising a zeolite and at least one subsequent        catalyst bed containing a second alkylation catalyst comprising        a zeolite, the first alkylation catalyst being more active for        the ethylation of benzene than the second alkylation catalyst,    -   b) introducing a feedstock of benzene and ethylene into the        multistage alkylation reaction zone;    -   c) operating the multistage alkylation reaction zone at        temperature and pressure conditions in which the benzene is in a        vapor phase to cause vapor-phase ethylation of the benzene in        the presence of the first and second alkylation catalysts to        produce an alkylation-product comprising a mixture of        ethylbenzene and polyalkylated aromatic components; and    -   d) withdrawing the alkylation product from the multistage        alkylation reaction zone.

In yet another embodiment, this invention can be either of the processesabove with the additional steps of separating the polyalkylated aromaticcomponents from the alkylation product and supplying at least a portionof the polyalkylated aromatic component along with benzene to atransalkylation reaction zone operated in the vapor or liquid phaseunder temperature and pressure conditions sufficient to causetransalkylation of the polyalkylated aromatic fraction to produce atransalkylation product having an enhanced ethylbenzene content and areduced polyalkylated aromatic components content.

In a further alternative embodiment, the invention can be any of theprocesses above, further including the steps of separating thepolyalkylated aromatic components from the alkylation product; andsupplying at least a portion of the polyalkylated aromatic component tothe alkylation reaction zone to cause transalkylation of thepolyalkylated aromatic fraction to produce a transalkylation producthaving an enhanced ethylbenzene content and a reduced polyalkylatedaromatic components content.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the general configuration of a reactor containing fourcatalyst beds.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

Aromatics alkylation reactions are highly exothermic, and many reactionschemes have been developed to control temperature rise through thereactor in an effort to minimize byproduct formation. One such solutionhas been the interstage introduction of lower temperature aromaticreactant feed streams to both act as a reactant and a to reduce thetemperature by producing a quenching effect. In most current vapor phasearomatics alkylation reaction systems, multiple beds of the samecatalyst are used sequentially for alkylaromatic production. Forexample, in a typical ethylbenzene reactor which operates with betweenfour and eight catalyst beds, only about 50 to 65% of the benzene feedcan be directed towards the top bed (reactor inlet). The remainingbenzene is added between the catalyst beds as a heat sink fortemperature control (quenching) purposes, since each bed operatesoptimally at essentially the same inlet temperature. This mode ofoperation generally causes the lowest and highest benzene to ethyleneratios to occur in the top and bottom beds respectively. The lowerbenzene flow in bed 1 of the reactor leads to higher by-productsformation due to reduced localized benzene to ethylene ratio and highertemperature rise across the catalyst bed.

This invention provides for an improved process for the production ofalkylaromatics by contacting the reactants in a reaction zone maintainedunder such conditions that the reaction occurs in the vapor phase and inthe presence of at least two catalysts exhibiting different activity.This allows the reaction zone, comprising a series of catalyst beds, tooperate at more varied temperatures and can reduce or eliminate the needfor quenching or otherwise cooling the effluent from each stage.

Arranging the catalyst in the alkylation reactor such that the highestactivity catalyst is in one or more upper bed(s) and the lowest activitycatalyst is in one or more later bed(s) allows the beds to operate atmore varied inlet temperatures. It would now be preferable to have arising temperature profile as the aromatic compound, for examplebenzene, and the alkylating agent, for example ethylene, flow throughthe reactor.

FIG. 1 shows a simplified four-bed reactor, in which the feed to thefirst catalyst bed 1 is a mixture of the aromatic reactant and thealkylating agent. Additional alkylating agent 2 and optionallyadditional aromatic reactant 8 are combined with the effluent from thefirst bed and introduced to the second catalyst bed. Again, additionalalkylating agent 4 and optionally additional aromatic reactant 10 arecombined with the effluent from the second bed and introduced to thethird catalyst bed. The same steps are repeated with additionalalkylating agent 6 and optionally additional aromatic reactant 12 beingcombined with the effluent from the third catalyst bed and introduced tothe fourth catalyst bed. The effluent from the fourth catalyst bed 14comprises a mono-alkylated aromatic compound, unreacted aromaticreactant, and poly-alkylated aromatic compounds. In the processes of theinvention, at least the first bed, and optionally up to the first threebeds contain a higher activity alkylation catalyst, and the input ofaromatic reactant is shifted from inlets 8, 10, and/or 12 to inlet 1.

In one embodiment where the aromatic hydrocarbon reactant is benzene andthe alkylating agent is ethylene, this catalyst arrangement would allowmore of the benzene, preferably greater than 60 wt. % of the total, morepreferably greater than 80 wt.%, and most preferably 100%, to bedirected toward the top (first) bed, resulting in an increased localizedbenzene to ethylene ratio and reduced temperature rise across the uppercatalyst bed in a downflow arrangement. This improvement would result inboth improved process yield and reduced byproduct formation,particularly reduced formation of byproducts not easily transalkylatedto form ethylbenzene.

In one embodiment of the present invention, highest activity catalyst isloaded into the top (first) bed(s) and lowest activity catalyst isloaded into the bottom bed(s). This allows the top bed to operate at thelowest inlet temperature and the catalyst temperature increasesprogressively as the reactants move towards the lower beds. Thiscatalyst arrangement requires little or no quench benzene to be addedinterstage between the beds for temperature control purposes. The quenchbenzene can be diverted towards the top catalyst bed, thereby increasingboth the overall (benzene and ethylene) weight hourly space velocity(WHSV) and the localized benzene to ethylene ratio through the uppercatalyst beds. This benzene shift provides a severity compensation forthe higher activity catalyst beds and also reduces the temperatureincrease across these beds due to a lower ethylene concentration.Product purity is improved by operating at a higher localized benzene toethylene ratio, higher overall WHSV, and lower temperatures both at theinlet and within the catalyst beds. Reduced decline in catalyst activityrelative to throughput is an additional unexpected benefit of thisinvention.

In an alternative embodiment, staging a higher activity catalyst in afirst catalyst bed with a lower activity catalyst in a second catalystbed can also be used to increase the capacity of a process configurationwithout increasing the overall aromatic reactant circulation and theassociated increased production of undesirable byproducts.

Catalysts suitable for vapor phase alkylation of aromatics include avariety of molecular sieves, particularly aluminosilicate zeolites,which are classified by framework type and described by the StructureCommission of the International Zeolite Association according to therules of the IUPAC Commission on Zeolite Nomenclature. A framework-typedescribes the topology and connectivity of the tetrahedrally coordinatedatoms constituting the framework and makes an abstraction of thespecific properties for those materials. Molecular sieves for which astructure has been established are assigned a three letter code and aredescribed in the Atlas of Zeolite Framework Types, 5th edition,Elsevier, London, England (2001), which is herein fully incorporated byreference.

Other molecular sieves include those described in R. Szostak, Handbookof molecular Sieves, Van Nostrand Reinhold, New York, N.Y. (1992), whichis herein fully incorporated by reference.

Molecular sieves preferred for use in the catalysts of this inventionare those having intermediate pore sizes, preferably having a poredimension from about 5 Angstroms to about 7 Angstroms. Examples ofsuitable molecular sieve materials for use in the alkylation catalystsof this invention include, but are not limited to, ZSM-5, described inU.S. Pat. No. 3,702,886; ZSM-11, described in U.S. Pat. No. 3,709,979;ZSM-12, described in U.S. Pat. No. 3,832,449; ZSM-35, described in U.S.Pat. No. 4,016,245; and ZSM-38, described in U.S. Pat. No. 4,046,859.

In practicing a particular desired chemical conversion process, it maybe useful to incorporate any of the above-described crystalline zeoliteswith a matrix or binder comprising another material resistant to thetemperature and other conditions employed in the process.

Useful matrix materials include both synthetic and naturally occurringsubstances, as well as inorganic materials such as clay, silica and/ormetal oxides. The latter may be either naturally occurring or in theform of gelatinous precipitates or gels including mixtures of silica andmetal oxides. Naturally occurring clays which can be composited with thezeolite include those of the montmorillonite and kaolin families, whichfamilies include the sub-bentonites and the kaolins commonly known asDixie, McNamee-Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite, oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment, or chemicalmodification.

In addition to the foregoing materials, the zeolites employed herein maybe composited with a porous matrix material, such as alumina,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-beryllia, and silica-titania, as well as ternary compositions,such as silica-alumina-thoria, silica-alumina-zirconia,silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may bein the form of a cogel. The relative proportions of zeolite componentand inorganic oxide gel matrix, on an anhydrous basis, may vary widelywith the zeolite content ranging from between about 1 to about 99percent by weight and more usually in the range of about 5 to about 80percent by weight of the dry composite.

Activity of a catalyst can be impacted by various factors including thesynthesis method, silica/alumina ratio, selection of binder, shape ofthe extruded particles, steaming, and other treatments. One measurementof relative activity of catalysts for certain kinds of reactions is thealpha value. Catalytic activity of zeolites, such as ZSM-5, is oftenreported using alpha value, which compares the catalytic crackingactivity of the catalyst (rate of normal hexane conversion per volume ofcatalyst per unit time) with the activity of a standard silica-aluminacracking catalyst. The alpha test is described in U.S. Pat. No.3,354,078; in the Journal of Catalysis, Vol. 4, p. 527 (1965); Vol. 6,p. 278 (1966); and Vol. 61, p. 395 (1980). The experimental conditionsof the test used herein include a constant temperature of 538° C. and avariable flow rate as described in detail in the Journal of Catalysis,Vol. 61, p. 395.

For aluminosilicate zeolites such as ZSM-5, the alpha value generallydecreases with increasing silica/alumina ratio in the synthesizedzeolite. Preferred silica/alumina ratios for the catalysts of thepresent invention are less than 200: 1, more preferably less than 100:1,and most preferably from about 12:1 to about 80:1. Other variables whichhave been found to impact the activity of the zeolite are the crystalsize and the selection of binder material. The alpha value of anas-synthesized zeolite or a catalyst composition can be reduced for aspecific application through a variety of treatment methods.

Alpha value is a better indicator of catalyst activity for somereactions than for others. For the purposes of this invention, catalystswould be selected for a particular alkylation reaction based on theiroverall suitability for that reaction. An alternative measurement ofcatalyst activity suitable for use in this invention would be acomparison of conversion between catalysts at a given base set ofoperating conditions for the reaction to be conducted. Appropriatecomparisons can be made based on conversion rates at the least severeoperating conditions appropriate for either of the catalysts beingcompared. The catalyst with higher conversion at the test conditionswould be the more active catalyst and would therefore be selected forthe initial bed(s) of the reactor.

Preferably the catalyst used in the first bed(s) of the reactor would beat least 10% more active than the catalyst used in a subsequent bed,based on a comparison of conversion rates at the operating conditions ofthe first bed of the reactor. Even more preferably, the catalyst used inthe first bed(s) would be at least 20% more active for the givenreaction than the catalyst used in a subsequent bed. Catalyst selectionswith 25%, 50%, 75%, 100%, and greater than 100% difference in activitywould be useful in this invention.

In one embodiment, the same zeolite would be used for the catalyst ineach of the beds, but that zeolite would be treated so as to alter itsactivity. For example, different binders could be used for formulatingthe catalyst used in the different beds. Silica used as a binder hasbeen found to result in a higher activity catalyst for aromaticsalkylation than alumina. Another example would be steaming or otherwisereducing the activity from the “as-synthesized” level to two differentlevels, and using the higher activity catalyst for the initial bed(s) inthe reactor followed by one or more beds containing the lower activitycatalyst. For example, an as-synthesized ZSM-5 may have a very highalpha value, but two batches of the same zeolite could be treated toreduce the respective alpha values such that the alpha value of thefirst is approximately double the alpha value of the second.

Alternatively, two different zeolites could be used so long as they wereselected to place the higher activity formulated catalyst in the initialbed(s) of the reactor.

It will be recognized that the surprising results herein originate fromthe concept of staging the catalysts by relative activity levels andthat this effect will be obtained with one or more beds of higheractivity catalyst followed by one or more beds of lower activitycatalyst regardless of the actual number of beds of each catalyst or inthe reactor as a whole. Although the examples contained herein refer toa first and second alkylation catalyst for ease of description, it willbe recognized by those skilled in the art that this invention appliesequally to the use of more than two levels of catalyst activity.

A catalyst suitable for use as the first alkylation catalyst of theinvention would be a molecular sieve suitable for use in aromaticsalkylation processes, preferably a molecular sieve bound with silica.The first alkylation catalyst would preferably be treated to reduce thealpha number from an as-synthesized alpha value but would generallystill have a relatively high alpha number.

An example of one catalyst suitable for use as the first catalyst wouldbe a silica bound ZSM-5 zeolite with relatively high alpha activitycompared to that generally preferred for the specific operatingconditions.

A preferred first catalyst would be approximately 65% ZSM-5 bound withapproximately 35% SiO2. Preferably the ZSM-5 is in the form of smallcrystals, preferably less than about 0.08 micron in diameter. TheSiO2/Al2O3 ratio of the ZSM-5 would be less than about 200:1, preferablyfrom about 5:1 to about 200:1, more preferably from about 20:1 to about100:1, and most preferably from about 50:1 to about 75:1. The SiO2binder is preferably comprised of between 10 and 90% colloidal silicasol such as Ludox HS-40, more preferably about 50% colloidal silica sol,and between 10 and 90% precipitated silica such as Ultrasil, morepreferably about 50% precipitated silica. The catalyst would preferablybe prepared by extruding the ZSM-5 with the colloidal silica sol andprecipitated silica (water and NaOH can be added to facilitate theextrusion) and drying the extrudate. A preferred shape is {fraction(1/16)}-inch cylindrical extrudates.

In one preferred embodiment, the dried extrudate is then humidified witha steam/air mixture and is exchanged with 1N ammonium nitrate to removesodium. The exchange is followed by a water wash with deionized water.The exchange/wash procedure is preferably repeated. The catalyst wouldthen be dried, calcined to about 600 to 1200° F. (about 315 to 650° C.),preferably about 1000° F. (about 538° C.), preferably in nitrogenfollowed by a mixture of air and nitrogen. The first catalyst would thenbe steamed to reduce the alpha activity to an alpha value from about 60to about 200, preferably from about 70 to about 100.

A suitable second catalyst could comprise the same ZSM-5 zeolite,preferably prepared by extruding the ZSM-5 with alumina (water can beadded to facilitate the extrusion) to {fraction (1/16)}-inch cylindricalextrudates, drying the extrudate, calcining the dried extrudate to about600 to 1200° F. (about 315 to 650° C.), preferably about 1000° F. (about538° C.), in nitrogen followed by a mixture of air and nitrogen. Thesecond catalyst could then be steamed to reduce the alpha activity to avalue less than that of the first catalyst, preferably less than about60, preferably from about 35 to about 55.

While the selection of different binder materials and steaming todifferent endpoints was described above, it will be recognized by thoseof ordinary skill in the art that any selection or treatment methodsuitable for staging the relative activity of aromatics alkylationcatalysts will fall within the scope of this invention.

One embodiment of this invention includes reduction and/or eliminationof interstage benzene addition. The use of a higher activity catalyst inthe first bed(s) allows for conversion using a lower temperature feed.This would then allow reduction of the interstage quench, furtherreducing the temperature increase in the first bed(s). Both thereduction in temperature and the increased B/E ratio reduce theproduction of unwanted byproducts. Alternatively, it may be possible toreduce the overall B/E ratio to the reactor as a whole, thus reducingoperating costs associated with recycling aromatics such as benzene backto the process.

In another embodiment, this invention can be a process for thevapor-phase reaction of ethylene with benzene, in a molar ratio ofbenzene to ethylene from about 5 to about 25, preferably 6 to 7, in amulti-stage alkylation reaction zone having a plurality ofseries-connected catalyst beds, preferably from 4 to 8 beds. Thereaction zone would comprise at least one catalyst bed containing afirst alkylation catalyst comprising a molecular sieve, preferably boundwith silica binder, and having an alpha value from about 60 to about200, preferably from about 70 to about 100, and at least one subsequentcatalyst bed containing a second alkylation catalyst with an alpha valuefrom about 10 to about 60. The reaction zone would be operated atalkylation conditions including temperature and pressure conditions inwhich the benzene is in a vapor phase to produce an alkylation productcomprising a mixture of ethylbenzene and polyalkylated aromaticcomponents. The alkylation product would be withdrawn from themultistage alkylation reaction zone, the polyalkylated aromaticcomponents would be separated from the alkylation product; and at leasta portion of the polyalkylated aromatic component would be suppliedalong with benzene to a transalkylation reaction zone operated in thevapor or liquid phase under temperature and pressure conditionssufficient to cause transalkylation of the polyalkylated aromaticfraction to produce a transalkylation product having an enhancedethylbenzene content and a reduced polyalkylated aromatic componentscontent.

A further alternative embodiment would involve alkylation of thearomatic component as described above, except that at least a portion ofthe separated polyalkylated aromatic components would be recycled to thealkylation reaction zone to cause transalkylation of the polyalkylatedaromatic fraction to produce a product having an enhanced ethylbenzenecontent and a reduced polyalkylated aromatic components content.

EXAMPLES

In order to provide a better understanding of the present inventionincluding representative advantages thereof, the following examples areoffered. Examples 1 and 2 will describe the use and performance of twoindividual catalyst compositions in ethylbenzene production. Example 3will demonstrate the performance of a reactor using the catalysts ofExamples 1 and 2 in sequential beds, and Example 4 will describe theimpact of using the design of example 3 with increased throughput.Examples 5, 6, and 7 will provide the results of the individualcatalysts in a six-bed reactor and a simulation of the results expectedfrom the application of this invention to that reactor.

Example 1 (Comparative) Single Catalyst

A catalyst was prepared by extruding ZSM-5, having an average crystalsize less than 0.08 micron and a SiO2/Al2O3 ratio of approximately 60:1,with alumina (water is added to facilitate the extrusion) to {fraction(1/16)}-inch cylindrical extrudates having approximately 35% alumina,drying the extrudate, calcining the dried extrudate to approximately 1000° F. (about 540° C.) in nitrogen followed by a mixture of air andnitrogen. The catalyst was then steamed to reduce the alpha activity tobetween 35 and 55.

This catalyst was loaded into all four beds of a four-bed reactor asshown in FIG. 1. Ethylene and benzene were introduced into the reactorwith an overall weight ratio of benzene to ethylene (B/E) of 21.6 and aWHSV of 70.8 hr⁻¹ based on the combined throughput of benzene andethylene. The details of the percent of total ethylene input at eachstage, the percent of total benzene input at each stage, the inlettemperature at each stage, the resulting product impurity levels, andthe decline in catalyst activity are shown in Table 1. It is noted thatoverallethylene conversion using fresh catalyst is generally in the 99.8to 99.95 weight % range.

Example 2 (Comparative) Single Higher Activity Catalyst

A second catalyst was prepared using the same type of ZSM-5 withapproximately 35% silica binder (approximately 50% Ludox HS-40, acolloidal silica sol, with approximately 50% Ultrasil, a precipitatedsilica. The catalyst was prepared by extruding the ZSM-5 with theUltrasil and Ludox (water and NaOH were added to facilitate theextrusion) to {fraction (1/16)}-inch cylindrical extrudates, drying theextrudate. The dried extrudate was then humidified with a steam/airmixture and exchanged with 1N ammonium nitrate to remove sodium. Theexchange was followed by a water wash with deionized water. Theexchange/wash procedure was repeated. The catalyst was then dried,calcined to approximately 1000° F. (about 540° C.) in nitrogen followedby a mixture of air and nitrogen. The catalyst was then steamed toreduce the alpha activity to between 70 and 100.

This catalyst was loaded into all four beds of a four-bed reactor asshown in FIG. 1. Ethylene and benzene were introduced into the reactorwith an overall weight ratio of benzene to ethylene (B/E) of 22.0 and aWHSV of 71.9 hr⁻¹ based on the combined throughput of benzene andethylene. For Example 2, the ethylene and benzene feed rate are shown asa percentage of the feed rates in Example 1. Details of the percent oftotal ethylene input at each stage, the percent of total benzene inputat each stage, the inlet temperature at each stage, the resultingproduct impurity levels, and the decline in catalyst activity are shownin Table 1.

It is noted that Example 2, using the higher activity catalyst, reflectslower concentrations of xylenes, DEB, and heavies in the ethylbenzeneproduct.

Example 3 Staged Catalyst Beds with Constant Throughput

For the purpose of Example 3, the catalyst of Example 2 was loaded intothe first 2 beds of the reactor, and the catalyst of Example 1 wasloaded into the subsequent 2 beds of the reactor. Again, throughput washeld roughly constant with the feed rates again shown as percentages ofthe feed rates represented by Example 1. The results of thisconfiguration are shown in Table 1. It is noted that, surprisingly, theresulting impurities are significantly lower than those of eithercatalyst alone.

Example 4 Staged Catalyst Beds with Increased Throughput

In Example 4, the reactor was loaded with catalyst as in Example 3, butthroughput of both ethylene and benzene were increased by 17.2% and 3.9%respectively as compared to Example 1. Surprisingly, this increase inthroughput did not result in significantly higher impurities or a higherdecline in catalyst activity than those shown in Example 1.

The surprising benefits of this invention can either be utilized toimprove product purity or to increase reactor capacity. A 10 to 15%increase in reactor capacity has significant economic benefits. Anothersurprising result is that the catalyst aging rate, expressed in terms ofdecline in % conversion per month is lower for the combination ofcatalysts shown in Example 3. In Example 4, the aging rate was higher,but even with significantly higher throughput, the aging rate was not ashigh as the weighted average of the rates experienced by either catalystalone. TABLE 1 Alkylation of Benzene in a 4-Bed Reactor Example 1 2 3 4Ethylene Feed Rate 100.0 99.8 100.6 117.2 Benzene Feed Rate 100.0 101.6100.6 103.9 WHSV (hr⁻¹⁾ 70.8 71.9 71.3 74 Overall B/E (wt.) 21.6 22.021.6 19.1 Bed 1 B/E (wt.) 53.2 56.6 76.3 61.3 Bed 1 Ethylene (%) 27.627.8 21.8 23.2 Bed 2 Ethylene (%) 27.2 28.7 27.0 26.2 Bed 3 Ethylene (%)27.2 28.7 30.9 27.6 Bed 4 Ethylene (%) 17.9 14.9 20.3 22.9 Bed 1 Benzene(%) 68.0 71.7 77.0 74.5 Bed 2 Benzene (%) 10.9 9.8 9.5 12.6 Bed 3Benzene (%) 10.9 9.8 7.0 6.8 Bed 4 Benzene (%) 10.2 8.8 6.5 6.2 Bed 1Inlet (° C.) 404 388 376 371 Bed 2 Inlet (° C.) 390 391 380 376 Bed 3Inlet (° C.) 383 393 390 385 Bed 4 Inlet (° C.) 396 396 399 391 p- &m-Xylene (ppm) 1660 1500 1360 1700 o-Xylene (ppm) 480 420 380 500 DEB/EB(wt. %) 26.65 24.55 21.8 25.6 Heavies (ppm) 3100 2600 2200 2900 EthyleneConversion 0.018 0.01 0.009 0.012 Decline (wt. %/mo.)

Examples 5, 6, and 7

Examples 5 and 6 provide actual data for each of the catalysts ofExamples 1 and 2 respectively when used in a six-bed reactor. Example 7provides a hypothetical example of the staged activity combination ofcatalysts of this invention when applied to a six-bed reactor, withthree beds of the more active catalyst followed by three beds of theless active catalyst. The data for these three examples are presented inTable 2 for comparison. TABLE 2 Alkylation of Benzene in a 6-Bed ReactorExample 5 6 7 Ethylene Feed Rate 100.0 100.6 101.7 Benzene Feed Rate100.0 100.2 101.1 WHSV (hr⁻¹) 40.0 39.9 40.3 Overall B/E (wt.) 17.3 17.217.2 Bed 1 B/E (wt.) 67.5 90.9 105.7 Bed 1 Ethylene (%) 12.6 10.7 10.4Bed 2 Ethylene (%) 13.8 13.9 13.5 Bed 3 Ethylene (%) 15.5 15.5 16.8 Bed4 Ethylene (%) 17.3 17.3 18.0 Bed 5 Ethylene (%) 19.4 21.1 20.4 Bed 6Ethylene (%) 21.3 21.4 21.0 Bed 1 Benzene (%) 49.2 56.6 63.7 Bed 2Benzene (%) 6.7 5.0 4.7 Bed 3 Benzene (%) 9.0 10.0 6.8 Bed 4 Benzene (%)10.4 8.0 7.1 Bed 5 Benzene (%) 11.6 10.8 9.1 Bed 6 Benzene (%) 13.0 9.58.4 Bed 1 Inlet (° C.) 372 365 354 Bed 2 Inlet (° C.) 378 366 359 Bed 3Inlet (° C.) 382 371 364 Bed 4 Inlet (° C.) 383 377 369 Bed 5 Inlet (°C.) 385 380 379 Bed 6 Inlet (° C.) 388 381 388 p- & m-Xylene (ppm) 410390 320 o-Xylene (ppm) 110 100 80 DEB/EB (wt. %) 10.5 9.0 8.0 Heavies(ppm) 2300 2000 1650 Ethylene Conversion 0.015 0.008 0.007 Decline (wt.%/mo.)

1. A process for alkylating an aromatic hydrocarbon reactant with analkylating agent to produce an alkylated aromatic product, said processcomprising: (a) introducing said aromatic hydrocarbon reactant and saidalkylating agent into a reactor unit containing a plurality ofsequentially arranged beds comprised of a first bed containing a firstcatalyst effective for alkylating said aromatic hydrocarbon reactant anda second bed downstream from said first bed and containing a secondcatalyst effective for alkylating said aromatic hydrocarbon reactant andhaving less catalytic activity than said first catalyst; (b) alkylatingin said first bed under alkylation conditions said aromatic hydrocarbonreactant with said alkylating agent to form a first effluent comprisinga mono-alkylaromatic compound, an unreacted portion of the aromatichydrocarbon reactant, and polyalkylated aromatic compounds, (c)alkylating in said second bed under alkylation conditions at least aportion of said unreacted aromatic hydrocarbon reactant present in saideffluent with said alkylating agent to form a product effluent, and d)removing said product effluent from said reactor unit, said producteffluent comprising a mono-alkylaromatic compound, an unreacted portionof the aromatic hydrocarbon reactant, and polyalkylated aromaticcompounds.
 2. The process of claim 1, wherein said alkylation conditionswithin the reaction zone comprise temperature and pressure conditions atwhich the aromatic hydrocarbon reactant is in a vapor phase.
 3. Theprocess of claim 1, wherein the molar ratio of the aromatic hydrocarbonreactant to the alkylating agent is from about 5 to about
 25. 4. Theprocess of claim 1, wherein the aging rate of the staged combination ofthe first and second catalysts is less than the aging rate of eithercatalyst individually.
 5. The process of claim 1, wherein the firstcatalyst has an alpha value greater than the alpha value of the secondcatalyst.
 6. The process of claim 1, wherein the first catalyst has analpha value from about 60 to about 200 and the second catalyst has analpha value from about 20 to about
 100. 7. The process of claim 1,wherein the reactor unit comprises from 4 to 8 catalyst beds.
 8. Theprocess of claim 1, wherein the first and second catalysts each comprisethe same molecular sieve.
 9. The process of claim 1, wherein the firstand second catalysts each has a crystal size of less than one micron.10. The process of claim 1, wherein the first catalyst comprises amolecular sieve and a silica binder and the second catalyst comprises amolecular sieve and an alumina binder.
 11. The process of claim 1,wherein the aromatic hydrocarbon reactant comprises benzene and thealkylating agent comprises ethylene.
 12. The process of claim 11,wherein at least 65% of the total benzene introduced to the reactor unitis introduced in the first bed of the reactor.
 13. The process of claim1, wherein the first catalyst is at least 10% more active than thesecond catalyst for alkylation of the aromatic hydrocarbon reactant atthe operating conditions of the first bed.
 14. The process of claim 13,wherein the first catalyst is at least 25% more active than the secondcatalyst for alkylation of the aromatic hydrocarbon reactant at theoperating conditions of the first bed.
 15. The process of claim 14,wherein the first catalyst is at least 50% more active than the secondcatalyst for alkylation of the aromatic hydrocarbon reactant at theoperating conditions of the first bed.
 16. The process of claim 15,wherein the first catalyst is at least 100% more active than the secondcatalyst for alkylation of the aromatic hydrocarbon reactant at theoperating conditions of the first bed.
 17. A process for the vapor-phaseethylation of benzene comprising: a) providing a multi-stage alkylationreaction zone having a plurality of series-connected catalyst beds, atleast one of the series-connected catalyst beds containing a firstalkylation catalyst comprising a zeolite and at least one subsequentcatalyst bed containing a second alkylation catalyst comprising azeolite, the first alkylation catalyst being more active for theethylation of benzene than the second alkylation catalyst, b)introducing benzene and ethylene into the multistage alkylation reactionzone; c) operating the multistage alkylation reaction zone attemperature and pressure conditions in which the benzene is in a vaporphase to cause vapor-phase ethylation of the benzene in the presence ofthe first and second alkylation catalysts to produce an alkylationproduct comprising a mixture of ethylbenzene and polyalkylated aromaticcomponents; and d) withdrawing the alkylation product from themultistage alkylation reaction zone.
 18. The process of claim 17,wherein the feedstock has a benzene/ethylene molar ratio from about 5 toabout
 25. 19. The process of claim 17, wherein the zeolite in the firstcatalyst has a silica/alumina ratio from about 5 to about
 200. 20. Theprocess of claim 17, wherein the zeolite in the second catalyst has asilica/alumina ratio from about 5 to about
 200. 21. The process of claim17, wherein the multistage alkylation reaction zone comprises 4 to 8catalyst beds.
 22. The process of claim 17, wherein the zeolite of thefirst and second alkylation catalysts each has a crystal size of lessthan one micron.
 23. The process of claim 17, wherein the firstalkylation catalyst is at least 25% more active than the secondalkylation catalyst for the ethylation of benzene at the operatingconditions of the first bed of the reaction zone.
 24. The process ofclaim 23, wherein the first alkylation catalyst is at least 50% moreactive than the second alkylation catalyst for the ethylation of benzeneat the operating conditions of the first bed of the reaction zone.
 25. Aprocess for the vapor-phase reaction of ethylene with benzene, theprocess comprising: a) introducing benzene and ethylene, in a molarratio of benzene to ethylene from about 5 to about 25 into a multi-stagealkylation reaction zone having a plurality of series-connected catalystbeds, at least one catalyst bed containing a first alkylation catalystcomprising a molecular sieve bound with silica binder and having analpha value from about 60 to about 200 and at least one subsequentcatalyst bed containing a second alkylation catalyst with an alpha valuefrom about 10 to about 60; b) operating each stage of the alkylationmultistage reaction zone at temperature and pressure conditions in whichthe benzene is in a vapor phase to produce an alkylation productcomprising a mixture of ethylbenzene and polyalkylated aromaticcomponents; c) withdrawing the alkylation product from the multistagealkylation reaction zone; d) separating the polyalkylated aromaticcomponents from the alkylation product; and e) supplying at least aportion of the polyalkylated aromatic component along with benzene to atransalkylation reaction zone operated in the vapor or liquid phaseunder temperature and pressure conditions sufficient to causetransalkylation of the polyalkylated aromatic fraction to produce atransalkylation product having an enhanced ethylbenzene content and areduced polyalkylated aromatic components content.
 26. The process ofclaim 25, wherein the reaction zone comprises from 4 to 8 catalyst beds.27. A process for the vapor-phase reaction of ethylene with benzene, theprocess comprising: a) introducing benzene and ethylene, in a molarratio of benzene to ethylene from about 5 to about 25 into a multi-stagealkylation reaction zone having a plurality of series-connected catalystbeds, at least one catalyst bed containing a first alkylation catalystcomprising a molecular sieve bound with silica binder and having analpha value from about 60 to about 200 and at least one subsequentcatalyst bed containing a second alkylation catalyst with an alpha valuefrom about 10 to about 60; b) operating each stage of the alkylationmultistage reaction zone at temperature and pressure conditions at whichthe benzene is in a vapor phase to produce an alkylation productcomprising a mixture of ethylbenzene and polyalkylated aromaticcomponents; c) withdrawing the alkylation product from the multistagealkylation reaction zone; d) separating the polyalkylated aromaticcomponents from the alkylation product; and e) supplying at least aportion of the polyalkylated aromatic component to the alkylationreaction zone of step (a) to cause transalkylation of the polyalkylatedaromatic fraction to produce a transalkylation product having anenhanced ethylbenzene content and a reduced polyalkylated aromaticcomponents content.
 28. The process of claim 27, wherein the reactionzone comprises from 4 to 8 catalyst beds.